1. Field of the Invention
The present invention relates to turbo machines, and in particular relates to a turbo machine being able to prevent from instability in flow, by suppressing swirl due to recirculation flow at an inlet of an impeller and by suppressing rotation stalls of the impeller, irrespective of the types and the fluid thereof.
In more details, the present invention relates to the turbo machines, such as for a pump, a compressor, a blower, etc., having non-volume type impeller therein, and in particular, relates to the turbo machine being able to prevent from the instability in flow, by suppressing a swirl or pre-whirl which is generated due to a main flow or component of the recirculation occurring at an inlet of an impeller and by suppressing rotation stalls thereof, thereby being suitable to be applied into a mixed-flow pump, which is used widely as water circulating pumps in a thermal power plant or in a nuclear power plant, or as drainage pumps, etc.
2. Description of Prior Art
Rotary machines being called by a name of "turbo machine" can be classified as below, depending upon the fluids by which the machines are operated and in types thereof.
1. With fluids by which the machine is operated:
Liquid, and Gas.
2. In Types:
An axial flow type, a mixed-f low type, and a centrifugal type.
Now, a mixed-flow pump is used mainly or widely due to easiness in operation thereof, and it comprises a suction casing, a pump and a diffuser, in a sequence from upper stream to down stream thereof.
A blade (of an impeller) rotating within a casing of the pump is rotationally driven on a rotary shaft, thereby supplying energy to the liquid which is suctioned from the suction casing. The diffuser has a function of converting a portion of velocity (or kinetic) energy of the liquid into static pressure.
A typical characteristic curve between a head and a flow rate of the turbo machine including the mixed-flow pump, where the horizontal axis shows a parameter indicating the flow rate while the vertical axis a parameter indicating the head, is as follows. Namely, it is common that the head falls down in the reverse relation to an increase of the flow rate in a region of low flow rate, however it has a characteristic of uprising at the right-hand side following the increase of the flow rate, during the time when the flow rate lies within a certain specific region. However, when the flow rate rises up further exceeding over the right-hand uprising region of the characteristic curve, the head begins to fall down, again, following the increase in the flow rate.
In a case where the turbo machine is operated with the flow rate of such the characteristic curve of uprising at the right-hand side, a mass of the liquid vibrates by itself, i.e., generating a surging phenomenon. It is believed that such the characteristic curve of uprising at the right-hand side is caused by, though the recirculation comes out at an outer edge of the inlet of the impeller when the flow rate flowing through the turbo machine is low, since at that instance, a flow passage or a channel for the liquid flowing into the impeller is narrowed and thereby generating a swirl in the liquid flowing into the impeller due to the influence of the recirculation mentioned above.
Since the surging gives damages not only upon the turbo machine, but also upon conduits or pipes which are connected to an upper-stream side and a down-stream side thereof, ordinarily, it is inhibited to be practiced in a region of low flow rate. Further, there were already proposed the following methods for suppressing the surging, other than an improvement made in the shape (i.e., profile) of the blade, for the purpose of expanding or enlarging the operation region of the turbo machine.
1. Casing treatment:
Thin or narrow grooves or drains, being from 10% to 20% of a chordal length of the blade, are formed in a casing region where the impeller lies, so as to improve a stall margin. Namely, with the casing treatment which were already proposed, the grooves being sufficient in the depth are formed in an inner wall (i.e., flow surface) of the casing in the region where the blades lie, in an axial direction, in a peripheral direction, or in an oblique direction, alternatively, in a radial direction or an oblique direction, respectively.
2. Separator:
A separator is provided for dividing the recirculation flow occurring at the outer edge of the inlet of the impeller into a reverse flow portion and a forward flow portion (i.e., in a main flow direction), in the region of low flow rate, thereby prohibiting the expansion of the recirculation. PA1 This is to suppress the generation of the swirl due to the recirculation by injecting or spouting out the high pressure fluid from an outside into a spot where the recirculation occurs.
As an example of a separator which is applied into the turbo machine of the axial flow type, in particular, there are proposed a suction ring type, a blade separator type, and an air separator type.
In the suction ring type, the reverse flow is enclosed within an outside of the suction ring, and in the blade separator type is provided a fin between the casing and the ring. Further, with the air separator type, a front end or a tip of the moving wing (i.e., the blade) is opened so as to introduce the reverse flows into the outside of the casing, thereby prohibiting the swirl from being generated due to the reverse flows by means of the fin. Thus, it is more effective, comparing with the former two types mentioned above, however, it comes to be large-scaled in the devices thereof.
3. Active control:
Furthermore, as an example of the conventional turbo machines, a mixed-flow pump will be described hereinafter. To a mixed-flow pump, it is required to show a head-flow rate characteristic curve (hereinafter, called by "head curve") having no behavior uprising at the right-hand side for enabling a stable operation, in a case where the pump is operated over the whole flow range thereof. However, ordinarily in a pump, it is common that the characteristics, such as an efficiency representing performance of the pump, a stability of the head curve, a cavitation performance, and an axial motive power for closure, etc., are in reversed relationships to one another. Namely, if trying to improve one of those characteristics, the other one(s) is is decreased down, therefore there is a problem that it is difficult to obtain improvements in at least two or more characteristics at the same time. For example, with a pump in which consideration was made primarily onto the efficiency thereof, the head curve shows a remarkable behavior uprising at the right-hand side in a portion thereof, thereby it has a tendency to be unstable.
For obtaining a head curve continuously falling down at the right-hand side for enabling the stable operation, in the conventional arts, as is mentioned in the above, it is already known that the casing treatment or the separator is provided or treated therein. Such the structure is already described, for example in U.S. Pat. No. 4,212,585.
Also, other than those, there is proposed a turbo machine, in which are formed plural pieces of grooves on the flow surface of the casing, for connecting between an inlet side of the impeller and an area or region of the flow surface of the casing where the blades reside, thereby obtaining a head curve having no such the characteristic of uprising at the right-hand side while suppressing the recirculation in the inlet thereof.
However, in accordance with the casing treatment and the separators of the prior arts mentioned above, although it is possible to shift the characteristic curve between head and flow rate including the portion uprising at the right-hand side into the lower flow rate side as it is, so as to expand the stable operation region thereof, however it is impossible to remove or cancel such the characteristic or behavior uprising at the right-hand side. Further, the turbo machine is decreased down by approximately 1% in the efficiency thereof, if it rises up by an every 10% in the stall margin, in accordance with the casing treatment.
Also, in such the active control, since there is a necessity to obtain the high pressure fluid from the turbo machine itself or an outside thereof, the efficiency of the turbo machine is decreased down as a whole system thereof.
Further, with a turbo machine, in which the grooves are formed for connecting between the inlet side of the impeller and the flow surface of the casing where the blades thereof reside, the processing of the grooves is easy and has a less decrease in an efficiency thereof, and it is also possible to obtain the head curve without such the uprising at the right-hand side in the characteristic thereof. However, there is not taken a consideration into a possibility that a fluctuation is generated in pressure due to interference between the flow from the blades of the impeller and the grooves when the blades pass by the plural grooves formed on the flow surface of the casing, thereby increasing vibration and noises.